1. Field of the Invention
Internal combustion engines are supplied with fuel by means of fuel injection systems that have a number of fuel injectors. Modern autoignition engines use high-pressure accumulator fuel injection systems. The fuel injectors, which can each be supplied with fuel by means of a high-pressure fuel accumulator (common rail), are triggered by means of solenoid valves or piezoelectric actuators. In fuel injectors whose actuating element is embodied in the form of a piezoelectric actuator, a needle-shaped injection valve member can be directly controlled by changing the electrical voltage supplied to the piezoelectric actuator. When the piezoelectric actuator is supplied with current, the piezocrystal stack undergoes a longitudinal extension that disappears again when the current is switched off.
2. Description of the Prior Art
In fuel injectors that are actuated by means of a piezoelectric actuator, the piezocrystal stack undergoes a longitudinal extension when supplied with current. Depending on the strength of the current supply, the piezocrystal stack of the piezoelectric actuator lengthens by a different amount. When the current is switched off again, though, the piezocrystal stack reverts back to its original length. It has turned out that supplying different current levels to the piezocrystal stack of a piezoelectric actuator is only able to achieve insufficiently stable intermediate positions of an injection valve member, which can be embodied in the form of a nozzle needle.
If a piezoelectric actuator is used in a fuel injector with a directly controlled injection valve member, then the required intermediate positions of the injection valve member between its completely open position and its completely closed position can only be maintained to a degree that is not sufficiently stable, which can be accompanied by a significant variation in the quantity of fuel injected into the combustion chamber of the autoignition engine in an intermediate position of the injection valve member.
In fuel injectors with direct needle control, in order to be able to open an injection valve member by means of a piezoelectric actuator, it is first necessary to overcome powerful forces. The injection valve member embodied in the form of a nozzle needle is usually pressed into its seat by system pressure, i.e. by rail pressure in the case of common rail injection systems. These forces can be on the order of 100 N. In order to be able to supply a sufficient flow of fuel through the fuel injector when the injection valve member is completely open, it is necessary for the injection valve member to execute a stroke of several hundred μm, e.g. on the order of between 200 and 300 μm. The values of the maximum opening force Fmax, which can be in the range of several hundred N, e.g. 400 N, and the maximum stroke distance of the injection valve member on the order of between 200 and 300 μm essentially determine the size of the piezoelectric actuator that is used to directly actuate the fuel injector.
By implementing a for example hydraulically functioning boosting action, it is in fact possible to vary the length/diameter ratio of the piezoelectric actuator; by itself, the volume of the actuator is determined by the maximum opening force Fmax to be overcome and by the maximum stroke distance to be executed by the injection valve member, which can be embodied in the form of a needle.